How can I give a single object multiple colors in cycles?
While you can assign a different material to each face on your model, I don't think that will give the result you want. I expect you want to use some noise to get colour variations, one way is to feed the noise into a colour ramp node.





There are other noise textures such as wave, voronoi, musgrave or magic that can give varying results, you may even want to mix a couple together. Just experiment with the scale, distortion and detail values to get a variation that you like.